Path MTU Discovery, Path MTU Discovery for IPv6 and the like are available as conventional path Maximum Transmission Unit (MTU) search techniques (see Non-Patent Document 1 and Non-Patent Document 2 below). In these techniques, a communication apparatus on the transmission side (sender) transmits a packet and confirms whether the packet is delivered successfully to the reception side (destination), where fragmentation of the transmitted packet by a network relay apparatus such as a router is prohibited.
According to these methods, if the packet transmitted from the transmission-side communication apparatus is larger in size than the path MTU, the packet cannot be transferred, and a network relay apparatus failing to the packet transfer transmits an error message to the sender. In other words, unless the transmission-side communication apparatus receives any error message, which means that any error message is not returned, the communication apparatus as the sender finds that the path MTU is larger in size than the transmitted packet length.
One of the merits to examine a path MTU resides in the effective usage of network resources. When a packet larger in size than the path MTU is transmitted, a network relay apparatus on the path divides a packet for fragmentation or an error message is transmitted so as to make the communication apparatus as the sender subdivide the packet and send the same again.
In case the network relay apparatus subdivides a packet on a path, packet fragmentation processing takes place, thus imposing a load on the network. Further, each of the subdivided packet includes a new header added thereto, thus degrading the transmission efficiency of data. When the network relay apparatus transmits an error message to the sender, the sender has to divide a packet with the same contents into small ones and transmit them again.
On the other hand, when a packet relatively smaller in size than the path MTU is transmitted, more packets in number are required to deliver the data with the same amount to a destination. Since each packet includes a header added thereto with a fixed size, a data region in each packet transmitted will be relatively made smaller, thus decreasing the transmission efficiency.
Thus, the transmission with a packet length as close as possible to the path MTU but not exceeding the path MTU can improve the transmission efficiency and can utilize network resources effectively.
As path MTU search methods other than the Path MTU Discovery disclosed in Non-Patent Documents 1 and 2, techniques disclosed in Patent Documents 1 to 3 have been known, for example.
Patent Document 1 discloses a technique in which, when a path MTU changes dynamically, a network side notifies a sender of the changed path MTU. According to the Path MTU Discovery disclosed in Non-Patent Documents 1 and 2, a sender needs to perform path search processing periodically so as to confirm whether a path MTU changes to a large value or not. In the techniques disclosed in Patent Documents 1 to 3, however, it is possible for the network side to notify a communication apparatus as the sender of the path MTU.
In Patent Document 1, when the network relay apparatus notifies the communication apparatus as the sender of the path MTU, Packet Too Big as an Internet Control and Management Protocol (ICMP) message is utilized similarly to the Path MTU Discovery disclosed in Non-Patent Documents 1 and 2. In Patent Document 2, when the network relay apparatus notifies the communication apparatus as the sender of the path MTU, a Binding Update (BU) message specified by mobile Internet Protocol (IP) described in the following Non-Patent Document 3 is utilized. In Patent Document 3, when the network relay apparatus notifies the communication apparatus as the sender of the path MTU, a method of adding an extension header (Destination Option Header) to a normal packet is utilized instead of using the specific messages as in Patent Document 1 and Patent Document 2.    Patent Document 1: Japanese Patent Application Publication No. 2004-48178    Patent Document 2: Japanese Patent Application Publication No. 2004-48181    Patent Document 3: Japanese Patent Application Publication No. 2005-57734 Non-Patent Document 1: Path MTU Discovery (RFC1191, November 1990)    Non-Patent Document 2: Path MTU Discovery for IPv6 (RFC1981, August 1996)    Non-Patent Document 3: Mobility Support in IPv6 (RFC3775, June 2004)
In the above-stated conventional techniques, however, in the case of a multipath state with a plurality of routing paths existing through which a packet can be delivered from a sender to a destination, there is a problem that a different path MTU cannot be used so as to effectively use network resources.
For instance, when the plurality of paths with different path MTUs exist and a network transfers packets using these plurality of paths, if a network communication apparatus as a sender knows a smaller path MTU value only, then smaller packets only will pass through a path with a larger path MTU among the plurality of existing paths from the viewpoint of the network side, which means that the network resources cannot be used effectively.
On the other hand, if the network communication apparatus as a sender knows a larger path MTU value only, then a packet with a larger packet length will pass through, and a packet will not pass through a path with a smaller path MTU so much (or in a path with a small path MTU, a packet subdivided by packet fragmentation processing will pass through), which also means that the network resources cannot be used effectively.
That is to say, according to the conventional path MTU discovery methods, in a multipath state with a plurality of routing paths existing through which a packet can be delivered from a sender to a destination, each path having a different path MTU, the network resources cannot be utilized effectively in spite of the existence of the plurality of paths.